1. Field of the Invention
This invention relates to surface acoustic wave devices such as filters and convolvers using special effects of surface acoustic waves, and particularly to surface acoustic wave devices that use KNbO3 single crystals having superior electromechanical coupling factors as piezoelectric layers.
This application is based on Patent Application No. Hei 11-80553 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In general, surface acoustic wave devices convert electric signals to surface acoustic waves propagating on surfaces of elastic substances, so that signals of specific frequencies are being extracted. Since scientists and engineers have discovered that surface acoustic waves can be subjected to excitation and signal reception efficiently on a piezoelectric substrate, they have studied and developed a variety of applications for signal function components such as filters and convolvers using the superior properties of surface acoustic waves, which are not provided by electromagnetic waves. So, the surface acoustic wave devices are used in a wide variety of fields in practice. Conventionally, the surface acoustic wave devices are manufactured by forming interdigital transducers, which function by transduction between electric signals and surface acoustic waves, on piezoelectric single crystals made of LiNbO3, LiTaO3, etc.
The surface acoustic wave device is designed such that an operational frequency f is determined based on an acoustic velocity v of surface acoustic waves propagating on a surface of an elastic substance and an electrode width w of an interdigital transducer in accordance with an equation (1), as follows:                     f        =                              v            _                    =                      y                          4              ⁢              w                                                          (        1        )            
where is a wavelength of a surface acoustic wave.
That is, as the electrode width w becomes small while the velocity v becomes large, the surface acoustic wave device can be used in higher frequencies. In order to obtain higher frequencies of a gigahertz order which is needed in future communications fields, however, it is necessary to select specific materials for elastic substances which allow propagation (or transmission) of surface acoustic waves at higher velocities because the present manufacturing techniques have limits in further narrowing electrode widths. An example of the material allowing propagation of acoustic waves at a high velocity is diamond. Japanese Unexamined Patent Publication No. Sho 64-62911 discloses an example of a surface acoustic wave device having a laminated structure in which a piezoelectric layer and an electrode layer are sequentially formed on a diamond layer.
Another factor which is required for selection of the material used for the surface acoustic wave device is an electromechanical coupling factor K2 representative of a capability of transduction between electric signals and surface acoustic waves. That is, it is necessary that the selected material has a large electromechanical coupling factor K2. As K2 becomes large, it is possible to obtain a surface acoustic wave device having a higher efficiency in transduction. From this point of view, the scientists and engineers have discovered that KNbO3, which is conventionally known as a ceramic material having piezoelectric properties, has an extremely large electromechanical coupling factor K2. Such a fact is confirmed through experiments. That is, results of the experiments show that a KNbO3 single crystal has a larger value of K2 than LiNbo3, which is conventionally believed to have a large electromechanical coupling factor K2. In particular, the KNbO3 single crystal at a specific crystal plane (e.g., plane (001)) in a specific direction (e.g., [100]) has an electromechanical coupling factor K2=0.053, which is approximately ten times greater than K2=0.055 in LiNbO3. This is disclosed in a monograph entitled xe2x80x9cSurface Acoustic Wave Substrate with Super High Electro-Mechanical Couplings Using KNbO3 Single Crystalxe2x80x9d written in a report of research no. 50 on pp. 27-31 (published on Nov. 27, 1996), which is provided for the No. 150 Symposium for SAW techniques at the Japan Academy Promotion Society. Reference is made to Japanese Unexamined Patent Publication No. Hei 10-65488, which discloses a surface acoustic wave substrate using potassium niobate (KNbO3) to obtain K2=0.5.
In addition, some documents disclose techniques for formation of KNbO3 thin films which are not always related to techniques of surface acoustic wave devices, as follows:
(1) Document 1: a monograph of PIONEER RandD Vol. 7 No. 1, entitled xe2x80x9cGrowth of Nonlinear Optical Crystal Films for SHG Devices by Vapor Phase Depositionxe2x80x9d, which discloses a method for forming KNbO3 thin films, used as waveguides of SHG (Second Harmonic Generation) light emission elements, on SrTiO3 substrates by using MOCVD (an abbreviation for xe2x80x9cMetal Organic Chemical Vapor Depositionxe2x80x9d).
(2) Document 2: a monograph of Mat. Res. Soc. Symp. Proc. Vol. 271 for 1992 Materials Research Society, entitled xe2x80x9cTHE GROWTH OF SINGLE CRYSTAL-LIKE AND POLYCRYSTAL KNbO3 FILMS VIA SOL-GEL PROCESSxe2x80x9d, which discloses a method for forming KNbO3 thin films on SrTiO3 substrates by sol-gel processing.
As described above, if the KNbO3 single crystals are used as piezoelectric materials, it is possible to actualize surface acoustic wave devices which have large electromechanical coupling factors K2 and high efficiencies in propagation of acoustic waves. In addition, it has been proven that values of K2 are changed in various manners in response to the propagation directions of surface acoustic waves in crystal structures of the KNbO3 single crystals. So, there is a strong demand to obtain a desired value of K2 or control K2 in the manufacture of surface acoustic wave devices. To cope with such a demand, it is necessary to match a propagational plane of the surface acoustic waves with a specific crystal orientation plane of KNbO3. That is, an interdigital transducer is formed to suit the specific crystal orientation plane, so that the propagational plane of the surface acoustic waves are matched with the specific crystal plane.
Suppose that a KNbO3 single crystal having a perovskite crystal structure shown in FIG. 8 is used as bulk material. To obtain a specific crystal orientation, it is necessary to perform very troublesome operations for cutting out a specific crystal plane from a KNbO3 single crystal whose crystal orientation is known. In addition, the KNbO3 single crystal is difficult to grow, and therefore is very expensive as an industrial material. For the reasons described above, it is very hard to use the aforementioned bulk material made of the KNbO3 single crystal as the material for the surface acoustic wave device.
The aforementioned difficulties challenge engineers to develop a concept in which the KNbO3 single crystal is not used as a bulk material but is used as a thin film being formed on some substrate in a laminated manner. However, even if a KNbO3 single crystal thin film is directly formed on general-purpose substrate materials such as MgO, Pt, Al2O3, GaAs and Si, it is impossible to obtain sufficient lattice matching between crystals, so KNbO3 is hardly subjected to epitaxial growth. In addition, the formed KNbO3 thin film itself is relatively low in crystallinity. As a result, it is impossible to obtain a good property (i.e., large value of K2). Both of the aforementioned documents 1, 2 are related to a method for forming a thin KNbO3 film directly on a SrTiO3 substrate. Even in such a method, it is impossible to obtain sufficient lattice matching between crystals of KNbO3 (lattice constants: a=5.70, b=5.72, c=3,97) and SrTiO3 (lattice constants: a=b=c=3.91).
It is an object of the invention to provide a surface acoustic wave device, which can be manufactured without troublesome operations for cutting out a specific crystal plane from a single crystal bulk material.
It is another object of the invention to provide a surface acoustic wave device which is increased in electromechanical coupling factor K2 to achieve a high efficiency in propagation of surface acoustic waves.
Thus, the surface acoustic wave device of this invention is used in wide bands and is manufactured with relatively low cost.
A surface acoustic wave device of this invention is basically configured by a substrates a buffer layer, a piezoelectric layer and an electrode layer. Herein, the substrate is made of a bulk material (e.g., SrTiO3) which allows the growth thereon of a perovskite compound crystal expressed by a general chemical formula of SrZO3 (where Z denotes an element such as Zr and Sn whose valence is 4). The buffer layer is formed on the substrate and is made of the perovskite compound crystal (e.g., SrZrO3, SrSnO3) which has: good lattice matching with KNbO3. The piezoelectric layer is made of a KNbO3 single crystal and is formed on the buffer layer with a thickness of 500 nm or so. The electrode layer is formed on or below the piezoelectric layer. An interdigital transducer consisting of input and output electrodes is formed by patterning using a photolithography technique being effected on the electrode layer, which is made of a metal material (e.g., Al). In addition, it is possible to form a temperature stabilization layer over the piezoelectric layer and/or electrode layer. The temperature stabilization layer is made of a material (SiO2) having a temperature coefficient reverse to a temperature coefficient of the piezoelectric layer so as to ease distortion caused by the differences in thermal expansion between the piezoelectric layer and the electrode layer. Incidentally, the temperature stabilization layer has a prescribed thickness, which is approximately 1000 nm.
Moreover, the KNbO3 single crystal forming the piezoelectric layer is made up with all crystal planes containing an X-axis. Further, the buffer layer is made of at least one crystal compound, which is selected from among SrTiO3, SrZrO3, SrMoO3, SrSnO3 and SrHfO3. Furthermore, the bulk material for the substrate is made of at least one crystal compound, which is selected from among SrTiO3, MgO, Pt, Al2O3, GaAs and Si.
Thus, it is possible to increase the electromechanical coupling factor K2, which contributes to broad-band application of the device to operate with high efficiency.: In addition, the surface acoustic wave device (e.g., filter and convolver) is capable of operating in a high frequency bandwidth and is manufactured with low cost.